The boundaries of Long Valley Caldera, in Eastern California, are sharply delineated by steeply dipping normal faults which, for the most part, are buried and obscured by recent volcanic and sedimentary cover. The only evidence for their presence is from their geophysical signatures on regional gravity, seismic and electromagnetic surveys. Recently, a surge of interest has focused studies in the western moat, where the volcanism is most recent, and where many workers feel that the geothermal potential is highest. A detailed, narrow-band (20--80 s) magnetotelluric profile in that area has delineated a parallel set of buried high angle normal faults which have virtually no expression at the surface. The outermost fault has a throw of approximately 500 m, while the innermost has a throw of approximately 2 km; neither has an expression in the surface morphology. Moreover, based on a relatively low resistivity contrast between the basement and the basin fill, we infer a relatively high fracture porosity in the Sierran basement, which suggests that there may be considerable lateral and vertical migration of meteoric water; a result which has considerable significance to understanding the regional hydrology. ¿ American Geophysical Union 1988